The amyloid precursor protein (APP) is intimately involved in the pathogenesis of Alzheimer's disease (AD). The AB peptide, a fragment of APP, accumulates in the neuropil and cerebral blood vessels of patients with AD and plays a critical role in the pathogenesis of the disease. However, the mechanisms by which AB exerts its deleterious effects have not been fully elucidated. Studies over the first funding period have established that AB causes a profound disruption of the mechanisms regulating the cerebral circulation. Thus, transgenic mice overexpressing mutated APP exhibit marked and selective alterations in endothelium-dependent vasodilation and in the cerebrovasodilation evoked by neural activity. These alterations are mediated by reactive oxygen species and occur in the absence of AB deposition in neuropil or blood vessels. In this renewal application, we will continue to study the mechanisms by which AB alters the regulation of the cerebral circulation. Three main hypotheses will be tested. First, that parenchymal and vascular deposition of AB worsens the cerebrovascular alterations observed in mice without AB deposition. Second, that cerebral blood vessels generate the free radicals mediating the dysfunction. Third, that the superoxide-producing enzyme NAD(P)H oxidase is a major source of the radicals responsible for the dysfunction. The reactivity of cerebral blood flow to pharmacological and physiological stimuli will be studied in mice equipped with cranial windows. First, we will study cerebrovascular responses in "young" and "old" APP mice to determine whether AB deposition in old mice worsens the dysfunction. Second, we will use markers of oxidative stress to pinpoint the cellular source(s) of the radicals responsible for the dysfunction. Third, we will study the cerebrovascular effects of synthetic All in mice lacking the gp9l phox subunit of NAD(P)H oxidase to determine whether this enzyme is involved. Fourth, we will study crosses between APP mice and gp9lphox-null mice to provide evidence that a NAD(P)H oxidase is also involved in the dysfunction induced by endogenous All in APP mice. These studies will continue to expand our understanding of the biological effects of AB and constitute a necessary step toward elucidating the contribution of vascular factors to the pathogenesis of AD.